The present invention relates to a four wheel steer vehicle having a suspension control system capable of varying a suspension characteristic such as suspension spring constant, vehicle height, suspension damping force or suspension geometry.
Japanese patent un-examined publication No. 59-81263 discloses a conventional four wheel steer vehicle. The vehicle of this example has a front wheel steering system, a rear wheel steering system, and a controller for controlling the rear wheel steering system in accordance with one of three steering modes such as an automatic control mode in which a characteristic of a rear wheel steer angle with respect to a front wheel steer angle is changed in accordance with driving condition, a fixed mode in which such a characteristic is fixed regardless of the driving condition, and a two wheel steer mode in which the rear wheel steer angle is fixed at zero regardless of the driving condition. The controller controls the rear wheel steer angle in accordance with one of three steering modes, by using the front wheel steer angle, vehicle speed, and rear wheel steer angle sensed by sensors. In this system, however, the rear wheel steer angle is determined without regard to a suspension characteristic. Therefore, this system is unable to maintain the desired vehicle characteristic of straight ahead driving and cornering when a change is made in front and rear suspension characteristics which affect the vehicle cornering characteristic and straight ahead driving characteristic.